This invention relates to x-ray systems, and more particularly to x-ray systems employing xenon detectors configured so as to be especially useful as airport luggage inspection devices or as foreign matter detection devices.
Fan beam x-ray systems employing gaseous medium detectors have most recently been employed in computerized tomography radiographic imaging systems which produce a shadow-free image of the internal structure lying in an imaginary plane passing through the patient being studied. These computerized tomographic imaging systems often employ gaseous medium detectors such as the xenon detectors described in U.S. Pat. No. 4,047,041 issued Sept. 6, 1977 to the applicant herein and assigned to the same assignee as this invention, which patent is hereby incorporated herein by reference. In these computerized tomography systems the x-ray fan beam is typically approximately 1 cm thick. In such systems, x-ray dosage received by the patient is a highly significant consideration in the structuring of the detector system as is the resolution sought to be achieved. Certain x-ray detectors employing gaseous media have also incorporated grid structures in addition to the electrode structure shown in the above-mentioned patent, the grid structure acting to accelerate electron and ion drift so as to increase the response time of the detector.
Also, present airport security systems employ flying spot scanner type x-ray systems for the inspection of luggage. Such flying spot x-ray systems are also used to inspect food for the presence of foreign matter, such as glass fragments in baby food jars. In these systems, a rotating collimator, with holes near its periphery, causes an x-ray pencil beam to sweep across the object. The next collimator hole produces the next scan, the linear velocity of the object (for example, on a conveyor belt) being adjusted relative to that of the rotating collimator so that succeeding scans are essentially contiguous to each other. In such a system, a single detector is used which is used to produce a video signal in a television-like format, that is, as a sequence of more or less contiguous scan lines. The image may then be displayed on a cathode ray tube (CRT) or analyzed so as to detect the presence of irregularities, such as foreign matter in a food container.
However, flying spot x-ray scanners are not satisfactory if low noise data is required at a moderate scan rate at, say for example, more than 1,000 picture elements per second. At such high scan rates, the x-ray source intensity requirements become very severe or completely impossible. The reason for this is obvious in that such flying spot scanners yielding a 100.times.100 element image of an object requires 10,000 times the total source x-ray energy compared to a standard x-ray picture in which the entire image is exposed simultaneously. Thus, a typical flying spot x-ray scanning system might require as long as 10 seconds to produce a square 10,000 picture element image. This 1,000 picture element per second or less requirement implies a linear velocity for the object of only 1 cm per second, assuming a 1 mm.times.1 mm picture element resolution and a 10 cm.times.10 cm field of view. Such object velocities are highly unsatisfactory for detecting foreign objects in processed food made in high speed production runs.
Other x-ray inspection systems, having lower power requirements, are also built using conventional phosphor screens for x-ray detection. In these systems, the x-rays are pulsed to expose the entire image simultaneously. A TV pickup tube (such as a vidicon) is optionally coupled to the phosphor screen through a lens. The vidicon target integrates the light and is subsequently scanned by an electron beam to yield an image. Such systems have relatively low source power requirements and can yield images suitable for visual inspection. However, vidicon target imperfections, scan noise, and other noise sources make vidicon systems unsatisfactory when low noise is required, as for example, if one wants to detect changes in x-ray intensity of only a few percent. Such relatively small changes in x-ray intensity, however do occur, for example when a small (1 mm.times.1 mm) glass particle occurs in a baby food jar.